Equation (7) of the original paper (Aasi et al. 2015) is in error; it should read

The discovery of the gravitational-wave (GW) source GW150914 with the Advanced LIGO detectors provides the first observational evidence for the existence of binary black hole (BH) systems that inspiral and merge within the age of the universe. Such BH mergers have been predicted in two main types of formation models, involving isolated binaries in galactic fields or dynamical interactions in young and old dense stellar environments. The measured masses robustly demonstrate that relatively “heavy” BHs ( 25  M) can form in nature. This discovery implies relatively weak massive-star winds and thus the formation of GW150914 in an environment with a metallicity lower than about 1/2 of the solar value. The rate of binary-BH (BBH) mergers inferred from the observation of GW150914 is consistent with the higher end of rate predictions ( 1  Gpc yr) from both types of formation models. The low measured redshift (z 0.1  ) of GW150914 and the low inferred metallicity of the stellar progenitor imply either BBH formation in a low-mass galaxy in the local universe and a prompt merger, or formation at high redshift with a time delay between formation and merger of several Gyr. This discovery motivates further studies of binary-BH formation astrophysics. It also has implications for future detections and studies by Advanced LIGO and Advanced Virgo, and GW detectors in space.

We present the results of the first search for gravitational wave bursts associated with high energy neutrinos. Together, these messengers could reveal new, hidden sources that are not observed by conventional photon astronomy, particularly at high energy. Our search uses neutrinos detected by the underwater neutrino telescope ANTARES in its 5 line configuration during the period January September 2007, which coincided with the fifth and first science runs of LIGO and Virgo, respectively. The LIGO-Virgo data were analysed for candidate gravitational-wave signals coincident in time and direction with the neutrino events. No significant coincident events were observed. We place limits on the density of joint high energy neutrino gravitational wave emission events in the local universe, and compare them with densities of merger and core-collapse events. Subject headings: gravitational waves — high energy neutrinos 1 Institut d’Investigació per a la Gestió Integrada de les Zones Costaneres (IGIC) Universitat Politècnica de València. C/ Paranimf 1 , 46730 Gandia, Spain. 2 CPPM, Aix-Marseille Université, CNRS/IN2P3, Marseille, France 3 GRPHE Institut universitaire de technologie de Colmar, 34 rue du Grillenbreit BP 50568 68008 Colmar, France 4 Technical University of Catalonia, Laboratory of Applied Bioacoustics, Rambla Exposició, 08800 Vilanova i la Geltrú, Barcelona, Spain 5 INFN Sezione di Genova, Via Dodecaneso 33, 16146 Genova, Italy 6 Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen Centre for Astroparticle Physics, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany 7 Direction des Sciences de la Matière Institut de recherche sur les lois fondamentales de l’Univers Service d’Electronique des Détecteurs et d’Informatique, CEA Saclay, 91191 Gif-surYvette Cedex, France 8 Nikhef, Science Park, Amsterdam, The Netherlands 9 APC, Université Paris Diderot, CNRS/IN2P3, CEA/IRFU, Observatoire de Paris, Sorbonne Paris Cité, 75205 Paris, France 10 LAM Laboratoire d’Astrophysique de Marseille, Pôle de l’Étoile Site de Château-Gombert, rue Frédéric Joliot-Curie 38, 13388 Marseille Cedex 13, France 11 INFN Sezione di Bologna, Viale C. Berti-Pichat 6/2, 40127 Bologna, Italy 12 Dipartimento di Fisica dell’Università, Viale Berti Pichat 6/2, 40127 Bologna, Italy 13 IFIC Instituto de F́ısica Corpuscular, Edificios Investigación de Paterna, CSIC Universitat de València, Apdo. de Correos 22085, 46071 Valencia, Spain 14 INFN -Sezione di Roma, P.le Aldo Moro 2, 00185 Roma, Italy 15 Dipartimento di Fisica dell’Università La Sapienza, P.le Aldo Moro 2, 00185 Roma, Italy 16 Clermont Université, Université Blaise Pascal, CNRS/IN2P3, Laboratoire de Physique Corpusculaire, BP 10448, 63000 Clermont-Ferrand, France 17 Géoazur Université de Nice Sophia-Antipolis, CNRS/INSU, IRD, Observatoire de la Côte d’Azur and Université Pierre et Marie Curie, BP 48, 06235 Villefranche-surmer, France 18 INFN Sezione di Bari, Via E. Orabona 4, 70126 Bari, Italy 19 INFN Laboratori Nazionali del Sud (LNS), Via S. Sofia 62, 95123 Catania, Italy 20 MIO, Mediterranean Institute of Oceanography, AixMarseille University, 13288, Marseille, Cedex 9, France; Université du Sud Toulon-Var, 83957, La Garde Cedex, France CNRS-INSU/IRD UM 110 21 Univ Paris-Sud , 91405 Orsay Cedex, France 22 Kernfysisch Versneller Instituut (KVI), University of Groningen, Zernikelaan 25, 9747 AA Groningen, The Netherlands 23 Direction des Sciences de la Matière Institut de recherche sur les lois fondamentales de l’Univers Service de Physique des Particules, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France 24 INFN Sezione di Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy 25 Dipartimento di Fisica dell’Università di Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy 26 University Mohammed I, Laboratory of Physics of Matter and Radiations, B.P.717, Oujda 6000, Morocco 27 Royal Netherlands Institute for Sea Research (NIOZ), Landsdiep 4,1797 SZ ’t Horntje (Texel), The Netherlands 28 Dr. Remeis-Sternwarte and ECAP, Universität ErlangenNürnberg, Sternwartstr. 7, 96049 Bamberg, Germany 29 Universiteit Utrecht, Faculteit Betawetenschappen, Princetonplein 5, 3584 CC Utrecht, The Netherlands 30 Universiteit van Amsterdam, Instituut voor Hoge-Energie Fysica, Science Park 105, 1098 XG Amsterdam, The Netherlands 31 Moscow State University, Skobeltsyn Institute of Nuclear Physics, Leninskie gory, 119991 Moscow, Russia 32 INFN Sezione di Catania, Viale Andrea Doria 6, 95125 Catania, Italy 33 Dipartimento di Fisica ed Astronomia dell’Università, Viale Andrea Doria 6, 95125 Catania, Italy 34 Département de Physique Nucléaire et Corpusculaire, Université de Genève, 1211, Geneva, Switzerland 35 Institute for Space Sciences, R-77125 Bucharest, Măgurele, Romania 36 IPHC-Institut Pluridisciplinaire Hubert Curien Université de Strasbourg et CNRS/IN2P3 23 rue du Loess, BP 28, 67037 Strasbourg Cedex 2, France 37 ITEP Institute for Theoretical and Experimental Physics, B. Cheremushkinskaya 25, 117218 Moscow, Russia 38 Dipartimento di Fisica dell’Università, Via Dodecaneso 33, 16146 Genova, Italy 39 Also at University of Leiden, the Netherlands 40 On leave of absence at the Humboldt-Universität zu Berlin 41 Also at Accademia Navale di Livorno, Livorno, Italy 42 LIGO California Institute of Technology, Pasadena, CA 91125, USA 43 California State University Fullerton, Fullerton CA 92831 USA 44 SUPA, University of Glasgow, Glasgow, G12 8QQ, United Kingdom 45 Laboratoire d’Annecy-le-Vieux de Physique des Particules 4 The ANTARES Collaboration, the LIGO Scientific Collaboration and the Virgo Collaboration (LAPP), Université de Savoie, CNRS/IN2P3, F-74941 AnnecyLe-Vieux, France 46 INFN, Sezione di Napoli, Complesso Universitario di Monte S.Angelo, I-80126 Napoli, Italy 47 Università di Napoli ’Federico II’, Complesso Universitario di Monte S.Angelo, I-80126 Napoli, Italy 48 Università di Salerno, I-84084 Fisciano (Salerno), Italy 49 LIGO Livingston Observatory, Livingston, LA 70754, USA 50 Cardiff University, Cardiff, CF24 3AA, United Kingdom 51 University of Sannio at Benevento, I-82100 Benevento, Italy and INFN (Sezione di Napoli), Italy 52 Albert-Einstein-Institut, Max-Planck-Institut für Gravitationsphysik, D-30167 Hannover, Germany 53 Leibniz Universität Hannover, D-30167 Hannover, Germany 54 VU University Amsterdam, De Boelelaan 1081, 1081 HV Amsterdam, the Netherlands 55 National Astronomical Observatory of Japan, Tokyo 181-8588, Japan 56 University of Wisconsin–Milwaukee, Milwaukee, WI 53201, USA 57 Università di Siena, I-53100 Siena, Italy 58 University of Florida, Gainesville, FL 32611, USA 59 LIGO Hanford Observatory, Richland, WA 99352, USA 60 University of Birmingham, Birmingham, B15 2TT, United Kingdom 61 Albert-Einstein-Institut, Max-Planck-Institut für Gravitationsphysik, D-14476 Golm, Germany 62 Montana State University, Bozeman, MT 59717, USA 63 European Gravitational Observatory (EGO), I-56021 Cascina (PI), Italy 64 Syracuse University, Syracuse, NY 13244, USA 65 LIGO Massachusetts Institute of Technology, Cambridge, MA 02139, USA 66 Columbia University, New York, NY 10027, USA 67 Stanford University, Stanford, CA 94305, USA 68 IM-PAN 00-956 Warsaw, Poland 69 Astronomical Observatory Warsaw University 00-478 Warsaw, Poland 70 CAMK-PAN 00-716 Warsaw, Poland 71 Bia lystok University 15-424 Bia lystok, Poland 72 NCBJ 05-400 Świerk-Otwock, Poland 73 Institute of Astronomy 65-265 Zielona Góra, Poland 74 The University of Texas at Brownsville, Brownsville, TX 78520, USA 75 San Jose State University, San Jose, CA 95192, USA 76 Moscow State University, Moscow, 119992, Russia 77 LAL, Université Paris-Sud, IN2P3/CNRS, F-91898 Orsay, France 78 ESPCI, CNRS, F-75005 Paris, France 79 NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA 80 University of Western Australia, Crawley, WA 6009, Australia 81 The Pennsylvania State University, University Park, PA 16802, USA 82 Université Nice-Sophia-Antipolis, CNRS, Observatoire de la Côte d’Azur, F-06304 Nice, France 83 Institut de Physique de Rennes, CNRS, Université de Rennes 1, 35042 Rennes, France 84 Laboratoire des Matériaux Avancés (LMA), IN2P3/CNRS, F-69622 Villeurbanne, Lyon, France 85 Washington State University, Pullman, WA 99164, USA 86 INFN, Sezione di Perugia, I-06123 Perugia, Italy 87 Università di Perugia, I-06123 Perugia, Italy 88 INFN, Sezione di Firenze, I-50019 Sesto Fiorentino, Italy 89 Università degli Studi di Urbino ’Carlo Bo’, I-61029 Urbino, Italy 90 University of Oregon, Eugene, OR 97403, USA 91 Laboratoire Kastler Brossel, ENS, CNRS, UPMC, Université Pierre et Marie Curie, 4 Place Jussieu, F-75005 Paris, France 92 University of Maryland, College Park, MD 20742 USA 93 Universitat de les Illes Balears, E-07122 Palma de Mallorca, Spain 94 University of Massachusetts Amherst, Amherst, MA 01003, USA 95 Canadian Institute for Theoretical Astrophysics, University of Toronto, Toronto, Ontario, M5S 3H8, Canada 96 Tsinghua University, Beijing 100084 China 97 University of Michigan, Ann Arbor, MI 48109, USA 98 Louisiana State University, Baton Rouge, LA 70803, USA 99 The University of Mississippi, University, MS 38677, USA 100 Charles Sturt University, Wagga Wagga, NSW 2678, Australia 101 Caltech-CaRT, Pasadena, CA 91125, USA 102 Pusan National University, Busan 609-735, Korea 103 Australian National University, Canberra, ACT 0200, Australia 104 Carleton College, Northfield, MN 55057, USA 105 The University of Melbourne, Parkville, VIC 3010, Australia 106 INFN, Sezione di Roma Tor Vergata, I-00133 Roma, Italy 107 Università di Roma Tor Vergata, I-00133 Roma, Italy 108 Università dell’Aquila, I-67100 L’Aquila, Italy 109 Instituto Nacional de Pesquisas Espaciais, 12227-010 São José dos Campos, SP, Brazil 110 The University of Sheffield, Sheffield S10 2TN, United Kingdom 111 Wigner RCP, RMKI, H-1121 Budapest, Konkoly Thege Miklós út 29-33, Hungary 112 Inter-University Centre for Astronomy and Astrophysics, Pune 411007, India 113 University of Minnesota, M

In Advanced LIGO, detection and astrophysical source parameter estimation of the binary black hole merger GW150914 requires a calibrated estimate of the gravitational-wave strain sensed by the detectors. Producing an estimate from each detector's differential arm length control loop readout signals requires applying time domain filters, which are designed from a frequency domain model of the detector's gravitational-wave response. The gravitational-wave response model is determined by the detector's opto-mechanical response and the properties of its feedback control system. The measurements used to validate the model and characterize its uncertainty are derived primarily from a dedicated photon radiation pressure actuator, with cross-checks provided by optical and radio frequency references. We describe how the gravitational-wave readout signal is calibrated into equivalent gravitational-wave-induced strain and how the statistical uncertainties and systematic errors are assessed. Detector data collected over 38 calendar days, from September 12 to October 20, 2015, contain the event GW150914 and approximately 16 of coincident data used to estimate the event false alarm probability. The calibration uncertainty is less than 10% in magnitude and 10 degrees in phase across the relevant frequency band 20 Hz to 1 kHz.

The gravitational-wave signal GW150914 was first identified on Sept 14 2015 by searches for short-duration gravitational-wave transients. These searches identify time-correlated transients in multiple detectors with minimal assumptions aboutthe signal morphology, allowing them to be sensitive to gravitational waves emitted by a wide range of sources including binary black-hole mergers. Over the observational period from September 12th to October 20th 2015, these transient searches were sensitive to binary black-hole mergers similar to GW150914 to an average distance of $\sim 600$ Mpc. In this paper, we describe the analyses that first detected GW150914 as well as the parameter estimation and waveform reconstruction techniques that initially identified GW150914 as the merger of two black holes. We find that the reconstructed waveform is consistent with the signal from a binary black-hole merger with a chirp mass of $\sim 30 \, M_\odot$ and a total mass before merger of $\sim 70 \, M_\odot$ in the detector frame.

On September 14, 2015, a gravitational wave signal from a coalescing black hole binary system was observed by the Advanced LIGO detectors. This paper describes the transient noise backgrounds used to determine the significance of the event (designated GW150914) and presents the results of investigations into potential correlated or uncorrelated sources of transient noise in the detectors around the time of the event. The detectors were operating nominally at the time of GW150914. We have ruled out environmental influences and non-Gaussian instrument noise at either LIGO detector as the cause of the observed gravitational wave signal.

We present an archival search for transient gravitational-wave bursts in coincidence with 27 single pulse triggers from Green Bank Telescope pulsar surveys, using the LIGO, Virgo and GEO interferometer network. We also discuss a check for gravitational-wave signals in coincidence with Parkes Fast Radio Bursts using similar methods. Data analyzed in these searches were collected between 2007 and 2013. Possible sources of emission of both short-duration radio signals and transient gravitational-wave emission include starquakes on neutron stars, binary coalescence of neutron stars, and cosmic string cusps. While no evidence for gravitational-wave emission in coincidence with these radio transients was found, the current analysis serves as a prototype for similar future searches using more sensitive second-generation interferometers.

On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-wave Observatory (LIGO) simultaneously observed the binary black hole merger GW150914. We report the results of a matched-filter search using relativistic models of compact-object binaries that recovered GW150914 as the most significant event during the coincident observations between the two LIGO detectors from September 12 to October 20, 2015. GW150914 was observed with a matched filter signal-to-noise ratio of 24 and a false alarm rate estimated to be less than 1 event per 203 000 years, equivalent to a significance greater than 5.1 σ.

ABSTRACT This article provides supplemental information for a Letter reporting the rate of (BBH) coalescences inferred from 16 days of coincident Advanced LIGO observations surrounding the transient (GW) signal GW150914. In that work we reported various rate estimates whose 90% confidence intervals fell in the range 2–600 Gpc−3 yr−1. Here we give details on our method and computations, including information about our search pipelines, a derivation of our likelihood function for the analysis, a description of the astrophysical search trigger distribution expected from merging BBHs, details on our computational methods, a description of the effects and our model for calibration uncertainty, and an analytic method for estimating our detector sensitivity, which is calibrated to our measurements.

We report results of a wideband search for periodic gravitational waves from isolated neutron stars within the Orion spur towards both the inner and outer regions of our Galaxy. As gravitational waves interact very weakly with matter, the search is unimpeded by dust and concentrations of stars. One search disk (A) is $6.87^\circ$ in diameter and centered on $20^\textrm{h}10^\textrm{m}54.71^\textrm{s}+33^\circ33'25.29"$, and the other (B) is $7.45^\circ$ in diameter and centered on $8^\textrm{h}35^\textrm{m}20.61^\textrm{s}-46^\circ49'25.151"$. We explored the frequency range of 50-1500 Hz and frequency derivative from $0$ to $-5\times 10^{-9}$ Hz/s. A multi-stage, loosely coherent search program allowed probing more deeply than before in these two regions, while increasing coherence length with every stage. Rigorous followup parameters have winnowed initial coincidence set to only 70 candidates, to be examined manually. None of those 70 candidates proved to be consistent with an isolated gravitational wave emitter, and 95% confidence level upper limits were placed on continuous-wave strain amplitudes. Near $169$ Hz we achieve our lowest 95% CL upper limit on worst-case linearly polarized strain amplitude $h_0$ of $6.3\times 10^{-25}$, while at the high end of our frequency range we achieve a worst-case upper limit of $3.4\times 10^{-24}$ for all polarizations and sky locations.

We present results from a search for gravitational-wave bursts coincident with two core-collapse supernovae observed optically in 2007 and 2011. We employ data from the Laser Interferometer Gravitational-wave Observatory (LIGO), the Virgo gravitational-wave observatory, and the GEO 600 gravitational-wave observatory. The targeted core-collapse supernovae were selected on the basis of (1) proximity (within approximately 15 Mpc), (2) tightness of observational constraints on the time of core collapse that defines the gravitational-wave search window, and (3) coincident operation of at least two interferometers at the time of core collapse. We find no plausible gravitational-wave candidates. We present the probability of detecting signals from both astrophysically well-motivated and more speculative gravitational-wave emission mechanisms as a function of distance from Earth, and discuss the implications for the detection of gravitational waves from core-collapse supernovae by the upgraded Advanced LIGO and Virgo detectors.

A transient gravitational-wave signal, GW150914, was identified in the twin Advanced LIGO detectors on 2015 September 2015 at 09: 50: 45 UTC. To assess the implications of this discovery, the detectors remained in operation with unchanged configurations over a period of 39 days around the time of the signal. At the detection statistic threshold corresponding to that observed for GW150914, our search of the 16 days of simultaneous two-detector observational data is estimated to have a false-alarm rate (FAR) of <4.9 x 10(-6) yr(-1), yielding a p-value for GW150914 of <2 x 10(-7). Parameter estimation follow-up on this trigger identifies its source as a binary black hole (BBH) merger with component masses (m(1), m(2)) = (36(-4)(+5), 29(-4)(+4))M-circle dot at redshift z = 0.09(-0.04)(+0.03) (median and 90% credible range). Here, we report on the constraints these observations place on the rate of BBH coalescences. Considering only GW150914, assuming that all BBHs in the universe have the same masses and spins as this event, imposing a search FAR threshold of 1 per 100 years, and assuming that the BBH merger rate is constant in the comoving frame, we infer a 90% credible range of merger rates between 2-53 Gpc(-3) yr(-1)(comoving frame). Incorporating all search triggers that pass a much lower threshold while accounting for the uncertainty in the astrophysical origin of each trigger, we estimate a higher rate, ranging from 13-600 Gpc(-3) yr(-1) depending on assumptions about the BBH mass distribution. All together, our various rate estimates fall in the conservative range 2-600 Gpc(-3) yr(-1).

In 2009-2010, the Laser Interferometer Gravitational-wave Observa- tory (LIGO) operated together with international partners Virgo and GEO600 as a network to search for gravitational waves of astrophysical origin. The sensitiv- ity of these detectors was limited by a combination of noise sources inherent to the instrumental design and its environment, often localized in time or frequency, that couple into the gravitational-wave readout. Here we review the performance of the LIGO instruments during this epoch, the work done to characterize the de- tectors and their data, and the effect that transient and continuous noise artefacts have on the sensitivity of LIGO to a variety of astrophysical sources.

Searches for a stochastic gravitational-wave background (SGWB) using terrestrial detectors typically involve cross-correlating data from pairs of detectors. The sensitivity of such cross-correlation analyses depends, among other things, on the separation between the two detectors: the smaller the separation, the better the sensitivity. Hence, a co-located detector pair is more sensitive to a gravitational-wave background than a non-co-located detector pair. However, co-located detectors are also expected to suffer from correlated noise from instrumental and environmental effects that could contaminate the measurement of the background. Hence, methods to identify and mitigate the effects of correlated noise are necessary to achieve the potential increase in sensitivity of co-located detectors. Here we report on the first SGWB analysis using the two LIGO Hanford detectors and address the complications arising from correlated environmental noise. We apply correlated noise identification and mitigation techniques to data taken by the two LIGO Hanford detectors, H1 and H2, during LIGO's fifth science run. At low frequencies, 40 - 460 Hz, we are unable to sufficiently mitigate the correlated noise to a level where we may confidently measure or bound the stochastic gravitational-wave signal. However, at high frequencies, 460-1000 Hz, these techniques are sufficient to set a $95%$ confidence level (C.L.) upper limit on the gravitational-wave energy density of \Omega(f)<7.7 x 10^{-4} (f/ 900 Hz)^3, which improves on the previous upper limit by a factor of $\sim 180$. In doing so, we demonstrate techniques that will be useful for future searches using advanced detectors, where correlated noise (e.g., from global magnetic fields) may affect even widely separated detectors.

We describe directed searches for continuous gravitational waves (GWs) in data from the sixth Laser Interferometer Gravitational-wave Observatory (LIGO) science data run. The targets were nine young supernova remnants not associated with pulsars; eight of the remnants are associated with non-pulsing suspected neutron stars. One target ' s parameters are uncertain enough to warrant two searches, for a total of 10. Each search covered a broad band of frequencies and first and second frequency derivatives for a fixed sky direction. The searches coherently integrated data from the two LIGO interferometers over time spans from 5.3-25.3 days using the matched-filtering. -statistic. We found no evidence of GW signals. We set 95% confidence upper limits as strong (low) as 4 x 10(-25) on intrinsic strain, 2 x 10(-7) on fiducial ellipticity, and 4 x 10(-5) on r-mode amplitude. These beat the indirect limits from energy conservation and are within the range of theoretical predictions for neutron-star ellipticities and r-mode amplitudes.

The Advanced LIGO gravitational wave detectors are second generation instruments designed and built for the two LIGO observatories in Hanford, WA and Livingston, LA. The two instruments are identical in design, and are specialized versions of a Michelson interferometer with 4 km long arms. As in initial LIGO, Fabry-Perot cavities are used in the arms to increase the interaction time with a gravitational wave, and power recycling is used to increase the effective laser power. Signal recycling has been added in Advanced LIGO to improve the frequency response. In the most sensitive frequency region around 100 Hz, the design strain sensitivity is a factor of 10 better than initial LIGO. In addition, the low frequency end of the sensitivity band is moved from 40 Hz down to 10 Hz. All interferometer components have been replaced with improved technologies to achieve this sensitivity gain. Much better seismic isolation and test mass suspensions are responsible for the gains at lower frequencies. Higher laser power, larger test masses and improved mirror coatings lead to the improved sensitivity at mid- and high- frequencies. Data collecting runs with these new instruments are planned to begin in mid-2015.

We report on an all-sky search for periodic gravitational waves in the frequency range 50–1000 Hz with the first derivative of frequency in the range −8.9 × 10−10 Hz s−1 to zero in two years of data collected during LIGO's fifth science run. Our results employ a Hough transform technique, introducing a χ2 test and analysis of coincidences between the signal levels in years 1 and 2 of observations that offers a significant improvement in the product of strain sensitivity with compute cycles per data sample compared to previously published searches. Since our search yields no surviving candidates, we present results taking the form of frequency dependent, 95% confidence upper limits on the strain amplitude h0. The most stringent upper limit from year 1 is 1.0 × 10−24 in the 158.00–158.25 Hz band. In year 2, the most stringent upper limit is 8.9 × 10−25 in the 146.50–146.75 Hz band. This improved detection pipeline, which is computationally efficient by at least two orders of magnitude better than our flagship Einstein@Home search, will be important for 'quick-look' searches in the Advanced LIGO and Virgo detector era.

Gravitational waves from a variety of sources are predicted to superpose to create a stochastic background. This background is expected to contain unique information from throughout the history of the Universe that is unavailable through standard electromagnetic observations, making its study of fundamental importance to understanding the evolution of the Universe. We carry out a search for the stochastic background with the latest data from the LIGO and Virgo detectors. Consistent with predictions from most stochastic gravitational-wave background models, the data display no evidence of a stochastic gravitational-wave signal. Assuming a gravitational-wave spectrum of Ω_{GW}(f)=Ω_{α}(f/f_{ref})^{α}, we place 95% confidence level upper limits on the energy density of the background in each of four frequency bands spanning 41.5-1726 Hz. In the frequency band of 41.5-169.25 Hz for a spectral index of α=0, we constrain the energy density of the stochastic background to be Ω_{GW}(f)<5.6×10^{-6}. For the 600-1000 Hz band, Ω_{GW}(f)<0.14(f/900  Hz)^{3}, a factor of 2.5 lower than the best previously reported upper limits. We find Ω_{GW}(f)<1.8×10^{-4} using a spectral index of zero for 170-600 Hz and Ω_{GW}(f)<1.0(f/1300  Hz)^{3} for 1000-1726 Hz, bands in which no previous direct limits have been placed. The limits in these four bands are the lowest direct measurements to date on the stochastic background. We discuss the implications of these results in light of the recent claim by the BICEP2 experiment of the possible evidence for inflationary gravitational waves.

During the LIGO and Virgo joint science runs in 2009-2010, gravitational wave (GW) data from three interferometer detectors were analyzed within minutes to select GW candidate events and infer their apparent sky positions. Target coordinates were transmitted to several telescopes for follow-up observations aimed at the detection of an associated optical transient. Images were obtained for eight such GW candidates. We present the methods used to analyze the image data as well as the transient search results. No optical transient was identified with a convincing association with any of these candidates, and none of the GW triggers showed strong evidence for being astrophysical in nature. We compare the sensitivities of these observations to several model light curves from possible sources of interest, and discuss prospects for future joint GW-optical observations of this type.